Voltage impulses occur in the eroding gap during the spark-eroding process, which impulses have a characteristic curve shape, namely the voltage generally increases first steeply to a voltage lying above the working voltage whereat the spark path is ionized, and drops to the actual working voltage after a specific ignition delay time, which substantially exists until the end of the impulse duration and drops then again to zero. The shape of the voltage impulses determines whether the impulse is usable or is in any way degenerated.
It is already generally known to measure, for example, the ignition voltage or the working voltage and the ignition delay time and to produce a control command when the measured values differ from the pregiven desired values, which control command controls the process data of the spark-eroding machine in such a manner that the cause for the deviations is eliminated.
However, it has been found, that during a normal process without interference, the ignition time delay does not constantly correspond with a specific, ideal desired value, but varies constantly due to the constantly changing gap conditions (Report No. 175-5/80, Lehrstuhl fuer Werkzeugmaschinen at the TH Aachen). A given desired value for the ignition delay time has then the result that during the normal process course, due to the constant deviations from the desired value, the electronic control constantly interferes with the process, which decisively worsens the efficiency of the process. The ignition delay time is, therefore, considered to be not suited as a process-controlling factor.
The purpose of the invention is to provide a method and a device for carrying out the method, which permits a use of the ignition delay time for controlling the process.
The invention utilizes the knowledge that in an interference-free eroding process never more than one specific number, for example three to five impulses of the same group are produced one after the other. Only when a tendency for process degeneration exists does the number of impulses of one group increase. The impulses are, according to the invention, sorted with respect to their ignition delay time or are associated with specific groups and are counted. If more than one pregiven limit number of impulses occurs one after the other in one group, then a control command is produced which changes the process data of the eroding machine in the sense of a normalization of the process. The groups are divided according to various aspects. A division which is too rough results in a process degeneration not being recognized or being recognized too late. A division which is too fine may possibly result in the system being too sensitive and most of all increases the technical requirements and thus the expense of the machine.
The invention provides that several impulse groups, with joining ignition delay ranges covering all together the entire possible ignition delay range, are formed so that a first impulse group covers an ignition delay range corresponding with or being larger than a normal impulse, a last impulse group covers a disappearing ignition delay range, and several impulse groups lying therebetween covers impulse delay ranges each lying therebetween. The control commands caused by the various impulse groups can basically be similar; however, there also exists the possibility to associate with the different impulse groups different control commands, since for example primarily the first impulse group and the last impulse group each indicate different tendencies of a process change.
Successive impulses of different impulse groups are inventively added up in a respectively associated counter, until either the limit number of impulses releasing the control command has been reached or the impulse sequence is interrupted by the occurrence of an impulse of another group. The counter is in the first case reset after giving the control command, in the second case directly again to an initial value.
The invention provides that the control commands each effect an interruption of the supply of voltage impulses to the working gap. This so called current-interruption strategy is generally sufficient to prevent a process degeneration. A development of the invention provides the control commands released upon reaching the limit number of impulses of the last impulse group effect a lifting off of the working electrode from the workpiece. The last impulse group with a disappearing ignition delay range is particularly critical and shows the tendency to form an arc, which is dealt with by the so called lifting strategy. By lifting off the electrode, the eroding process is, in a conventional manner, interrupted for a longer period of time and a flushing of dielectric is reinforced.
In a further development of the invention, the impulses of the first impulse group are not counted, and an exceeding of a limit number of impulses of this group remains without consideration. The first impulse group is characterized in having an ignition delay range corresponding with a normal impulse or being greater than a normal impulse. The impulses are not critical with respect to the quality of the machining task also when the ignition delay is greater compared with the normal impulse, at most the efficiency, not, however, the quality of the machining is affected.
The ignition delay time is measured inventively in such a manner that, with a rising edge and with a dropping edge of each impulse, the interval is fixed through a lower threshold value for the voltage and an upper threshold value for the voltage. The lower threshold value represents a minimum voltage for the working voltage, the upper threshold value a minimum voltage for the ignition voltage. It can be determined, on the one hand, from these intervals whether a working impulse has taken place, on the other hand, whether an ignition voltage has been reached and how great the ignition delay time is.
An impulse selection device with several outputs, each corresponding with one impulse group, is connected to the working gap of the eroding machine. The outputs each emit one counting impulse upon occurrence of an impulse of an associated impulse group. The counting impulses are each fed to counting devices which are connected after the impulse-selection device. The counting devices emit, upon reaching the limit number of impulses, a carry over signal, which are transformed into a control command for controlling the eroding machine in control devices inserted after the counting devices. The impulse selection is done by an evaluation of the voltage time intervals through the aforementioned threshold values. The time intervals are determined by a comparator defining a lower threshold value for the impulse voltage and a comparator defining the upper threshold value for the impulse voltage, which are connected before the impulse selection device.
The counting devices each inventively include a binary counter which can be reset and is connected to an output of the impulse-selection device to which, through an OR-gate, are fed the signals applied at the other outputs of the selection device as preset signals. The binary counter adds the signals coming from the selection device until either the limit number is reached, or until it is reset, through an impulse signal of another impulse group, to its initial value, as will be described in greater detail below.
One common timing circuit is connected after all counting devices, which timing circuit interrupts, with a carry over signal starting out from the counting devices, for a specific time the supply of impulses to the working gap. A synchronizing signal additionally fed to the timing circuit and coming from the eroding generator causes a cutoff command to extend to full operating impulses. Thus operating impulses which are too short and can effect increased electrode wear are avoided.
An additional timing circuit is preferably connected after the counting device associated with the fourth impulse group and is used to control the feed drive of the electrode in the sense of a quick gap enlargement.
In a further development of the invention, analogous luminous-band indicators for indicating the respectively successively counted impulses of each impulse group are connected to the counting devices. The luminousband indicator makes optically visible the maximum addition of impulses of the same group and thus the tendency toward process degeneration.